linux-kernel - Re: [PATCH] x86/mm: Do not verify W^X at boot up

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Date:   Tue, 25 Oct 2022 09:53:27 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH] x86/mm: Do not verify W^X at boot up

On Tue, Oct 25, 2022 at 3:16 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> This seems to boot...

This looks much better, thanks.

But this:

> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> @@ -801,8 +803,9 @@ void __init poking_init(void)
>         spinlock_t *ptl;
>         pte_t *ptep;
>
> -       poking_mm = copy_init_mm();
> -       BUG_ON(!poking_mm);
> +       __poking_mm = init_mm;
> +       mm_init(&__poking_mm, NULL, __poking_mm.user_ns);
> +       poking_mm = &__poking_mm;

Should probably be just

        poking_mm = mm_alloc();

because we shouldn't be messing with 'mm_init()' in places like this,
and we shouldn't be exporting it either:

> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1104,7 +1104,7 @@ static void mm_init_uprobes_state(struct mm_struct *mm)
>  #endif
>  }
>
> -static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> +struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,

since "mm_alloc()" would seem to be exactly what we need, and it's
what execve() already uses. It creates a new VM with nothing attached,
ready to be populated.

Or is there some reason why mm_alloc() doesn't work? It does require
"current" and "current->user_ns" to be set up, but that should be true
even during _very_ early boot.

                Linus